It is commonplace in the metal-working industry to subject metals to various heating operations, e.g., for annealing and/or stress relieving or in conjunction with hot working operations. For example, a steel billet or slab, prior to rolling into rod or sheet, is commonly placed in a heat-treating furnace in which it is uniformly heated. During such heating, the surfaces of the metal are especially liable to suffer the effects of oxidation phenomena, e.g., the formation of a layer of the metal oxide on the surface of the metal object and/or the selective oxidation of an alloying constituent of the metal. The latter phenomenon is particularly exemplified by the surface decarburization of steel or other ferrous alloys. The effects may be exhibited by the formation of an oxide scale or, in the case of decarburization, by the creation of a surface layer which has changed chemical and physical characteristics. If this layer is extensive, it subsequently must be removed. Metal losses incurred in these ways can be severe and, therefore, very costly. The high temperature treatment of ceramic substrates also often leads to deleterious degradation of the surface. This is evidenced by corrosion, erosion and the like. It is, therefore, of prime importance that some means should be found of shielding the surfaces from reactive atmospheres during any heat-treating operation.
It is well known that, in order to reduce these losses, small items may be subjected to their necessary heat treatment in a controlled inert or non-oxidizing atmosphere. Although this expedient has been fairly successful in many instances, expensive equipment is necessary, e.g., gas flow control apparatus and specially-constructed furnaces in which air may be prevented from entering. Furnaces in which the atmosphere can be controlled are suitable for heating small objects such as tools but it is costly to make use of such apparatus to heat large objects such as billets and slabs which may weigh several tons. Moreover, if the metal object is required to be removed from the furnace while still in a heated condition, fairly severe oxidation, decarburization or the like can still take place upon exposure to the atmosphere.
Alternative means of protecting the surfaces of metal articles to be heated have therefore been developed. Thus, it is known to apply a paint to the surfaces of the metal before subjecting it to heat treatment. Such paints may include materials which are preferentially oxidized (e.g., powdered aluminum, ferrosilicon or silicon) and are thus intended to function in a sacrificial manner. Other types of paint depend upon the physical exclusion of the atmosphere from the surface by the ability of their constituents to form a glaze when heated. Materials used in such paints are mixtures of various refractory oxides, slags, silica and ground glass. The preferentially oxidized materials mentioned above have also been included in the glaze-forming preparations. Varying degrees of success have been achieved by the use of these known paints which are generally applied to the metal surface as a layer of the order of 1/8 inch thick. However, none of them has been found to be capable of consistently reducing oxidation losses to a satisfactory degree although, in some instances, the amount of metal lost as oxide scale has been reduced by as much as 70%. A reduction in losses even of this magnitude is, however, considered unsatisfactory in that the degree of oxidation still suffered is inevitably accompanied, in the case of steels, by surface decarburization. Thus, a machining operation is necessary to remove the decarburized layer.
It may be observed in this connection that where a mechanical protection is to be achieved, as by a glaze, it is important that there should be no cracks or pinholes in the protective layer. Otherwise, the oxidation and/or decarburization effects tend to spread beneath the glaze far beyond the crack or pinhole itself.
There are other applications where a high-temperature paintable composition would be useful. For example, it would be desirable to have a coating that can be applied to devices from which asbestos-containing materials have been removed. It is very difficult to remove all residue of the asbestos; therefore, a paintable sealant could be utilized to encase the device to prevent the escape of this residual asbestos. Some applications will involve subsequent operation at elevated temperatures. Since large surface areas may be involved in this application, the coating material should have low cost.
Still another application for paintable coatings involves providing a protective surface for the base material (usually a ferrous metal) of ladles used for molten aluminum, magnesium, zinc, etc. This coating, to be effective, must be abrasion resistant, be relatively thick (0.005 to 0.125 inch), and be adequately adherent even during thermal cycling.
A very similar application is the forming of a coating on the interior surfaces of a "permanent" mold in, for example, the aluminum industry. This coating typically is up to 0.125 inch thick and must provide thermal insulation, quick release, etc.
Typical of prior art coatings is that described in U.S. Pat. No. 3,440,112 issued to F. E. G. Ravault on Apr. 22, 1969. This composition included silicon carbide, ferrosilicon, silica flour, bentonite, powdered glass and sodium or potassium cryolite to form a fusible glaze. Another of the prior art protective coatings is described in U.S. Pat. No. 3,861,938, issued to R. P. Jackson on January 21, 1975. The coating of this reference involves the use of an alkali-stabilized sol form of silica together with chromic oxide formed by the in situ oxidation of chromium metal.
U.S. Pat. No. 3,037,878, issued to R. J. Cowles, et al., on June 5, 1092, describes another coating composition for use in protecting metals against oxidation and/or decarburization during heat treatment. The composition involves oxides of aluminum, silicon and an alkali metal which fuse to form various crystalline phases. This coating is used at thicknesses of 1/64 to 1/32 inch (15.6 to 31.2 mils).
Another protective coating is described in U.S. Pat. No. 3,301,702, issued to S. L. Ames, et al., on Jan. 31, 1967. This coating, for which the surface must first be cleaned, consists of an alkali metal silicate and aluminum oxide.
A coating based upon sodium metaborate is described in U.S. Pat. No. 2,785,091, issued to C. A. M. Rex on Mar. 12, 1957; a coating based upon mica is described in U.S. Pat. No. 2,774,681, issued to P. Huppert, et al., on Dec. 18, 1956; and coatings based upon self-spalling ceramics are described in U.S. Pat. Nos. 3,459,601 and 3,459,602, issued to E. E. Muller on Aug. 5, 1969. Other patents generally related to protective coatings are: U.S. Pat. Nos. 3,399,078, issued to C. A. M. Bang on Aug. 27, 1968; 3,454,433, issued to E. E. Muller on July 8, 1969; 3,178,321, issued to W. R. Satterfield on Apr. 13, 1965; and 3,677,796, issued to R. T. Girard, et al., on July 18, 1972.
In general, the compositions of the above-cited prior art have certain drawbacks. Those compositions that form glass-like coatings at the temperatures of heat treatments often transfer a portion of the glaze to rolls of a rolling mill or to other handling apparatus. This type of glaze also can cause coated pieces to bond together when placed in contact with each other at a high temperature. This, of course, is detrimental to the equipment. Some coatings can only be removed using some form of substantial abrasion after the heat treatment step. For other compositions, numerous layers must be applied (and dried) before a thickness is achieved that will give satisfactory protection against oxidation and/or decarburization, etc. Furthermore, as discussed above, the surface of the item to be protected must be first cleaned before use of certain of the protective coatings.
The paintable composition described in our above-referenced U.S. Pat. No. 4,898,618, the content of which is included herein by reference, provides a suitable composition for most of the applications described above. However, several of the specific compositions disclosed therein are relatively expensive and therefore are precluded from applications where large quantities are required. Of the compositions in that patent application, the composition disclosed in Example 8 is a dilute composition of reduced cost; however, this composition is too expensive for some of the applications such as the asbestos sealant coating.
Accordingly, it is an object of the present invention to provide a paintable composition for use in the protection of a substrate, such as a metal object, against corrosion, oxidation and/or decarburization, etc., during heat treating steps at elevated temperatures to about 2400 degrees F.
It is another object to provide a coating composition for this protection that can be applied to the surface in thin coats (a few mils) and result in satisfactory protection.
Another object is to provide a protective coating composition that can be applied without previous treatment of the surface.
A further object of the present invention is to provide a protective coating composition which will not adversely affect apparatus used to process the item after the heat treating step or cause similarly coated pieces to bond together during heating.
It is also an object of the present invention to provide a coating composition that can be used on essentially all iron-based materials and, thus, is compatible with the various expansion behaviors of these materials.
An additional object of the present invention is to provide a paintable coating composition that provides a sealant to prevent the escape of environmental substances from a substrate.
Also, it is an object of the present invention to provide a paintable composition which can be applied to produce coatings of sufficient thickness to protect ladles used for molten metals and the like, such as molten aluminum, magnesium and zinc.
It is another object of the present invention to provide a paintable composition for various high temperature applications having a sufficiently low cost such that the composition can be used for coating extensive surfaces economically.
Additionally, it is an object of the present invention to provide a paintable composition that can be used to produce a coating of up to about 0.125 inch in molds used in casting of molten metals.
These and other objects of the present invention will become apparent upon a consideration of the full description of the invention which follows.